The IEEE standard 802.11a supplement to the IEEE standard 802.11 specification defines how to implement an orthogonal frequency division multiplexing (OFDM) system in the 5.15-5.25, 5.25-5.35, and 5.725-5.825 GHz unlicensed national information structure bands. As it happens, these frequency bands are shared by certain radar systems and so it is necessary, and required by the Federal Communications Commission, that all wireless communication devices utilizing these bands be able to sense the presence of a radar signal on a particular channel or channels and then quickly stop transmitting on the channel(s). Specifically, in a wireless LAN that includes a number of access points (APs) that transmit frames to and receive frames from wireless communications devices, it is required that master devices, which could be the APs for instance, are all capable of detecting the presence of radar signals in the 5 GHz frequency spectrum and notifying all of the slave devices, which in this case could be the wireless communications devices, that radar is present in one or more channels and that the slave devices should vacate those channels immediately. More specifically, prior to establishing a session with a slave wireless communications device, the master devices are required to sense the medium for some minimum, specified time period of time to determine whether radar signals are present in any particular channel(s). If no radar signal is detected in a particular channel, it is said that this channel is “available” to be utilized by the wireless communications device to establish a communications session. As mentioned above, if a master device detects a radar signal in any particular channel during the minimum, specified period of time, this channel is made “unavailable” and the master device is responsible for instructing all slave devices, associated with it, to immediately stop transmitting on this channel. Furthermore, a slave device is not permitted to transmit frames prior to receiving an enabling signal from a master device indicating that a channel is available for use. An enabling signal could be any frame of information transmitted by the master device such as a beacon or a probe response for instance.
The protocol described above is generally referred to as dynamic frequency selection (DFS) and it operates in the background of a wireless LAN so as to be largely transparent to the user of a wireless communications device as long as the device is stationary or only associated with one AP during an entire communications session. However, the continuity of a communications session between an AP and an associated wireless communications device can be interrupted in the event that the user of the wireless device moves around in space. Such movement is generally referred to as roaming and it results in the wireless communications devices having to disassociate with a current AP, whose transmission signal quality is deteriorating, and associate with another AP whose signal quality is better than that of the AP it is currently associated with.
The handoff process whereby a wireless communications device becomes disassociated from an AP it is currently communicating with and associates with another AP can be managed by either the AP or by the wireless communications device. Assuming that the process is managed by the wireless communications device, it monitors the signal quality of a communications link with the AP it is currently associated with and also monitors the signal quality of a signal or signals it receives on a different channel or channels from one or more other APs that are within transmission range and with which it is not currently associated. At the point in time that the signal strength of the current communications link falls below a threshold and the signal strength received from another AP with which it is not currently associated rises above a threshold, the handoff process can be initiated. In order for a wireless communications device to associate with a new AP, it is necessary to first conduct an authentication process and then an association process. At the point that the wireless communications device sends an association request to a new AP, the current AP with which the wireless device has a communications link is free to stop sending frames to the wireless device. At this point, it is essential that the new AP respond very quickly to the wireless communication device's request to associate and that the new communications link is established with a minimum of delay. In practice there are delays, but such delays are not typically noticeable or objectionable to the user.
Continuing to refer to the handoff process in more detail, a wireless communications device typically transmits probes on one or more channels other than the one it is currently transmitting on in order to receive probe responses from other APs with which it might associate that will permit a higher quality communications link. The operation of transmitting a probe and receiving a probe response for each channel typically occurs between periods of transmission and reception of voice or other packets to or from the AP with which the wireless device is currently associated. Unfortunately, sending such probes in an 802.11a environment is not allowed prior to the wireless device knowing that the channel on which it is sending the probe is available, or radar free. The FCC dictates that devices must sense the medium for available channels and this process takes time which the phone can't easily accommodate during a session. This means that either the wireless communications device is responsible to scan each channel before transmitting a probe for some minimum period of time before the handoff process can begin or that the AP is responsible for scanning the medium for available channels. Clearly, it is not practical for the wireless communications device to scan the medium, as this would cause an unacceptable delay in initiating a communications session.
Generally, an AP in a WLAN operating according to the standard 802.11 specification transmits two types of signals that contain all of the information that a wireless communications device needs in order to complete the hand off process. One is a management type frame called a beacon and the other is a control type frame called a probe response. As it is necessary for a wireless communications device to transmit a probe signal to an AP in order to receive a probe response from the AP, and as such activity is not permitted in channels shared with radar, it is necessary to implement some method whereby the wireless communications device can rely on a signal, such as a beacon, that can be received without having to actively request it. On the other hand, beacons are only transmitted by AP's infrequently, for instance every 102.4 msec, therefore, it is necessary for the wireless device to passively scan the medium, during periods not otherwise employed by the communications device to transmit or receive frames from the AP with which it is currently associated, for at least a period of time necessary to be certain that an entire beacon is received. In other words, the wireless communications device operates in a time-sliced manner to communicate with the AP with which it is currently associated and to scan all of the other channels, both those shared with radar and those not shared with radar, in order to receive a signal indicating that some other channels are available on which to communicate with another AP.
Furthermore, it is important from the perspective of the wireless communications device user that the communications session be of consistently high quality. That is, the audio quality is of high fidelity, that the transmitted or received frames of audio information are not clipped or dropped and that there is no noticeable or only minimal delay added as the result of the hand off process. In order to provide such a high quality communications session on a consistent basis, it is necessary to quantify the time it takes to scan other channels in order to determine which ones are available and of these which ones are of the highest quality.
In light of the above problems associated with the hand off procedure in a network that shares its frequency band with radar signals, it is desirable that the hand off of a wireless communications device from one AP to another will not result in any noticeable discontinuity in the communication session. Further, it is desirable that the amount of time it takes to complete the hand off of a wireless communications device session from one AP to another AP is predictably small and that the wireless communications device is able to utilize both regularly occurring and irregularly occurring AP signals in order to determine whether a channel is available.